I have noticed that HTML emails I send generate "HTML and text parts are different" as part of the content analysis, increasing the email's score by 1.5. Is anyone able to explain what this means exactly? That there is text etc outside of the < HTML > tags? Something else?

This rule was probably intended to get rid of so-called Bayes poison. Spammers were trying to send a HTML message, with completely uncorrelated gibberish in the text part of a multipart/alternative message, intended to confuse Bayes filtering. Apparently this rule is not working reliably for all mail clients or languages.

There are many people who for security reasons never view HTML mails. There is also a significant percentage of mail client software that is incapable of displaying HTML at all. So we can't help but recommend that you refrain from sending HTML mails in the first place.

Most mail clients do not send HTML formatted mails by default, and in Outlook you can turn it off.